A field within a structure can itself be a structured item composed
of other fields, other structures, or both. You can declare a
substructure in two ways:

By nesting structure declarations within other structure
or union declarations (with the limitation that you cannot refer
to a structure inside itself at any level of nesting).

One or more field names must be defined in the STRUCTURE
statement for the substructure, because all fields in a structure
must be named. In this case, the substructure is being used as a
field within a structure or union.

Field names within the same declaration nesting level must be
unique, but an inner structure declaration can include field
names used in an outer structure declaration without conflict.

By using a RECORD statement that specifies another
previously defined record structure, thereby including it in
the structure being declared.

See the example in the Section B.12.1 for
a sample structure declaration containing both a nested structure
declaration (TIME) and an included structure (DATE).